(4) The use of gas in artillery shell does not require especially trained troops inasmuch as gas shell are fired in the same manner as ordinary shell, and by the same gun crews. Moreover, since artillery gas shell are used generally only for ranges of a mile or more, the direction and velocity of the wind are of minor importance. Another factor which adds to the advantage of artillery shell in certain cases is the ability to land high concentrations of gas suddenly upon a distant target through employing a large number of the largest caliber guns available for firing gas. Notwithstanding the above named disadvantages of wave attacks it was felt by the Americans from the beginning that successful gas cloud attacks were so fruitful in producing casualties and were such a strain upon those opposed to it, that they would continue. Furthermore, since artillery shell contain about 10 per cent gas, while gas cylinders may contain 50 per cent, or even more of the total weight of the cylinder, the efficiency of a cloud gas attack for at least the first mile of the enemy’s territory is far greater than that of the artillery gas attack. It was accordingly felt that the only thing necessary to make cloud gas attacks highly useful and of frequent occurrence in the future was the development of mobile methods—methods whereby the gas attack could be launched on the surface of the ground and at short notice. For these reasons gas wave attacks may be expected to continue and to eventually reach a place of very decided importance in Chemical Warfare. GAS SHELL The firing of gas in artillery shell and in bombs has another great advantage over the wave attack just mentioned. There is a very great latitude in the choice of those gases which have a high boiling point or which, at ordinary temperatures, are solids. Mustard gas is an example of a liquid with a high boiling point, and diphenylchloroarsine an example of a gas that is ordinarily solid. For the above reason the term “gas warfare” was almost a misnomer at the close of the War, and today is true only in the sense that all the substances used are in a gaseous or finely divided condition immediately after the shell explode or at least when they reach the enemy. PROJECTOR ATTACKS Still another method of attack, developed by the British and first used by them in July, 1917, was the projector (invented by Captain Livens). This was used very successfully up to the close of the War, and though the German attempted to duplicate it, his results were never as effective. The projector consists of a steel tube of uniform cross section, with an internal diameter of about 8 inches. By using nickel steel the weight may be decreased until it is a one man load. The projector was set against a pressed steel base plate (about 16 inches in diameter) placed in a very shallow trench. [Illustration: FIG. 2.—Livens’ Projector. The Type shown is an 18 cm. German Gas Projector, captured during the 2d Battle of the Marne.]